Regulatory elements from the related spec genes of Strongylocentrotus purpuratus yield different spatial patterns with a lacZ reporter gene

The Spec1 and Spec2 genes of Strongylocentrotus purpuratus are closely associated with the differentiation of aboral ectoderm. To examine cis-regulatory elements involved in the spatial expression of the Spec genes, we fused the Escherichia coli lacZ gene containing a nuclear targeting signal to 5′ flanking DNA plus 5′ untranslated leader sequences from Spec1, Spec2a, and Spec2c. All three genes contain 700 bp of highly conserved DNA in their upstream regions, but in Spec1 and Spec2c large insertions interrupt the conserved regions. The Spec-lacZ reporter gene plasmids were microinjected into eggs of S. purpuratus, Lytechinus variegatus, and L. pictus, and β-galactosidase activity was determined in situ by X-gal staining. The Spec2a- lacZ fusion gene, which contained 1516 bp of 5′ flanking DNA and 18 bp of 5′ untranslated leader sequence, was preferentially expressed in aboral ectoderm cells in all three species. The Spec1- lacZ fusion gene was expressed in a strikingly different fashion-preferentially in primary and secondary mesenchyme cells, occasionally in aboral ectoderm cells, and less often in oral ectoderm and endoderm cells. The staining pattern was the same in either homologous or heterologous embryos. The Spec2c-lacZ fusion gene, like Spec2a-lacZ, was preferentially expressed in aboral ectoderm, but staining of other cell types was frequently observed. To further delineate sequences required for correct spatial expression, we deleted 800 bp of 5′ flanking DNA from the Spec2a-lacZ fusion gene, resulting in a ΔSpec2a-lacZ fusion gene that contained only the conserved DNA region. This gene fusion showed preferential expression in aboral ectoderm cells. However, the cell type specificity was not as great as with the parental Spec2a-lacZ plasmid. These experiments implied that the conserved DNA region, associated with all Spec genes examined, was insufficient for complete aboral ectoderm specificity, and suggested that a spatial repressor element existed between -1516 and -697 bp in the 5′ flanking DNA of Spec2a.